1. Field of the Invention
This invention relates to a method and apparatus for processing data representative of a visual representation, typically including a combination of text, graphics, and images, that is to be output to a visual-output device, such as a screen display or print device. More particularly it relates to such a method and apparatus in which a data memory, referred to herein functionally as a "buffer memory", has reduced capacity requirements resulting from the selective compression of data.
2. Related Art
The preferred embodiment of, and preferred method of practicing the present invention is directed to printers that form a raster image typically connected indirectly over a network, or directly to a computer for printing documents created on the computer. The invention is realizable for other forms of output devices as well, such as a video display generated on a CRT monitor or an LCD. Thus, the device creating the actual visual representation is referred to as a "visual-output device". The visual area within which the visual representation exists is referred to as a "page", regardless of its actual form. The complete visual representation is referred to as a "page representation". A separately defined part of a page representation is referred to as an "object".
One of the significant cost elements in a conventional printer is a buffer memory, also referred to as a frame buffer, for storing raster data defining the page representation. Conventional printer configurations employ buffer memories that are capable of storing all of the raster data required to define each pixel on a page. An extensive amount of memory capacity is therefore typically required. A black-and-white representation for a 8.5 inch.times.11 inch sheet of paper at a pixel density of 300 dpi (dots or pixels per inch) requires in excess of 1 MByte (1 million 8-bit bytes) of memory. Higher spatial and tonal resolutions, color printing and larger paper sizes require even more memory. A continuous tone, four-color representation at a pixel density of 600 dpi for the same sized page requires about 135 MBytes of memory. Since the printer costs rise with memory size, it is desirable to provide printers with reduced memory requirements.
A memory device known by the proprietary name of "Memory Miser" produced by Advanced Micro Devices of Santa Clara, Calif., stores data in a resident memory by applying a compression algorithm to all of the data input. When required for output it is decompressed based on the reverse of the compression algorithm and output. If used in a printer, such a device would reduce the amount of memory required. However, the memory would need to be at least large enough to store the most complex page representation in order to be able to process any page that is input. This printer would have little flexibility in processing the variety of page representations possible with present day printers.